Heating in the bedroom: How many degrees are comfortable at home?. According to consumer center.de, “at least 16 degrees” is best.
Federal Environment Agency: Temperatures in living areas should not be more than 20 degrees, as long as the temperature is perceived as “comfortable”. Leave the windows tilted at night - but not when the heat is on. You should also make sure there is enough fresh air. Shock ventilation is recommended during the heating season.